1. Field of the Invention
This invention relates to lens mountings, and more particularly to a mounting mechanism adapted to interchangeable lenses for single lens reflex cameras or the like.
2. Description of the Related Art
It has been known in the art to provide a mechanical mounting unit for a zoom lens having the auto-focus device incorporated therein in the interchangeable form to the single lens reflex camera.
For this kind of mechanical mounting unit, the zoom lenses have generally been so designed that the front lens group is axially moved forward to increase the focal length of the entire system by manipulating the zoom ring, while automatic focusing is performed by axially moving that front lens group with the help of an electric motor in such a way that the total focusing movement is far shorter than the total zooming movement.
There are known many lenses having motorized operating mechanisms for the diaphragm unit, or the focusing lens and the zoom components.
The motors incorporated within the casings of the conventional lens mounting units can be classified as the pencil (solid) type, i.e. iron core motors, coreless motors and brushless motors, and the hollow type known in Japanese Laid-Open Patent Application Nos. SHO 56-51328 and 56-147131.
The prior known techniques of incorporating the motor within the casing had, in the case of the former or pencil type, either to shape the casing with an awkward projection to accommodate the motor, or, for the prevention of such a projection from appearing, to increase one of the axial separations between the components of the optical system, or to unduly largely decrease the effective diameter of the rear component, producing the drawback that unless particular care was taken in the lens design, the bulk and size of the casing becomes large, and the production cost increases.
In the case of the latter or hollow type, because the rotor and stator of the motor are arranged on respective barrel members at the inner and outer surfaces confronting each other, a great increase in the complexity of mounting mechanism occurs.
In the prior known mechanical mounting unit for the above-described kind of auto-focus zoom lens, because the motor was located on the stationary member near the annular ferrule of the mounting unit, as the front lens group is at the front terminal end of zooming movement, the distance between the motor and the front lens group holder was so long that no gear train could be found as a driving torque transmission. As such means, use was made of a spline shaft. Another method is to employ a driven gear of extremely long face width.
In the prior art of zoom lens mountings besides the drawbacks that the driving torque transmission system was complicated in structure and of large scale, because the frictional power loss and the load torque of the transmission system were increased to an undesirably large extent, a large consumption of kinetic energy resulted. To compensate for this, a larger motor than necessary is deployed.
Therefore, the use of the conventional method of incorporating a motor in the zoom lens mounting unit leads to a serious problem that the bulk and size became larger with an increase in the weight and, moreover, the manufacturing cost and the operating cost are very high.